The present invention relates to a method for modifying surface properties of a shaped article of a polymeric material such as a vinyl chloride-based resin or a silicone rubber or, more particularly, to a method for reducing the bleeding phenomenon of plasticizers or other additive ingredients on the surface of the shaped article or for increasing the affinity of the surface to water by the treatment of the surface with low temperature plasma.
Needless to say, vinyl chloride-based resins belong to one of the most important classes of thermoplastic resins owing to their excellent and very versatile properties along with their relative inexpensiveness so that they are widely employed in a variety of application fields as shaped in various articles. In particular, rigidity or flexibility of shaped articles of vinyl chloride-based resins can be adequately controlled by formulating the resin with a plasticizer. The thus plasticized resins can give relatively flexible shaped articles so that plasticized resin compositions are fabricated into films, sheets, synthetic leathers, tubes, hoses, bags, coating materials and the like used in various fields such as medical instruments, packaging materials for foodstuffs, materials for agricultural use, building materials and the like.
Furthermore, it is a very common practice that vinyl chloride-based resins are formulated with various kinds of additive ingredients such as flame retardants, anti-oxidants, ultraviolet absorbers, lubricants and others according to the particular needs for the improvements of the workability of the resin composition as well as the properties of the articles shaped with the resin composition.
One of the most serious problems in these articles shaped not only with vinyl chloride-based resins but also with the other kinds of plastic polymers as well as in shaped articles of rubbery elastomers such as silicone rubbers formulated with the additive ingredients, typically, a plasticizer, is that the plasticizer contained in the shaped article may sometimes migrate toward the surface of the article and exude on the surface of the article in the long run resulting in inferior properties of the articles. This phenomenon is usually called "bleeding", prevention of which is one of the most important problems difficult to solve in the technology of synthetic resin and rubber processing.
Bleeding of the plasticizer and other additives is undesirable not only due to the deterioration of the properties of the shaped articles but also due to the transfer of the additives exuded on the surface of an article to the surface of the other body being in contact with the shaped article in question.
Along with the loss in the beauty of the surface appearance of the articles, bleeding or transfer of the additives is especially undesirable when the shaped article is used in a medical instrument or in contact with foods because safety of the exuded additives to the human health is not established in general so that the use of these plastic resins and rubbers in these fields is sometimes largely limited.
Various attempts have been made to solve the above described problem of surface bleeding of plasticizers and other additives. The methods hitherto proposed for the purpose include a method of treatment or irradiation of the surface of the articles with ionizing radiation, high-energy electron beams or ultraviolet light, a method of providing coating films on the surface of the articles with a suitable synthetic resin capable of preventing surface bleeding of the additives and a method of chemical treatment in which certain special chemicals are applied on to the surface of the articles or admixed in the composition of the resin or rubber for shaping the articles. These methods are, however, not satisfactory in most cases in the effectiveness with, instead, an adverse effect of impairing several advantageous properties inherently possessed by the resin or rubber per se.
For example, irradiation with ionizing radiation or high-energy electron beams may produce crosslinking between the polymer molecules not only in the superficial layer of the shaped article but also in the subsurface layer due to the excessively high energy of the radiation so that the flexibility of the articles is largely lost. Treatment with ultraviolet light is sometimes undesirable by the reason of coloring in the surface depending on the kind of polymer due to the degradation of the polymer molecules in the surface layer of the shaped article. The chemical means is not free from the problem of surface erosion of the shaped articles and the poor adhesivity and durability of the films provided on the surface of the article.
Another serious problem in the shaped articles of polymeric materials is the poor affinity of the surface with water. This nature of the polymer surface is especially detrimental when the article is to be used in contact with aqueous liquids such as the medical instruments used in contact with the body fluids. The affinity of the surface with water can be increased, of course, by treating the surface with surface active agents or by incorporating like compounds into the polymeric material before shaping into articles. Application of these methods is, however, limited because of the low durability of the effect or the adverse effects on the properties of the shaped articles.
Recently, it has been proposed that the surface bleeding of plasticizer and other additives in a shaped article of several kinds of resins and rubbers can be prevented by exposing the article to an atmosphere of low temperature plasma of a gas such as rare gases, oxygen, nitrogen, carbon monoxide and the like of low pressure generated by glow discharge. This method of plasma treatment is effective for preventing migration and surface bleeding of the additives by the formation of highly crosslinked layer only in the very surface of the article without affecting the desirable properties of the article as a bulk. It has been shown by the inventors that the plasma treatment of shaped articles of resins and rubbers is also effective in improving the wettability, adhesivity and printability as well as in reducing electrostatic charging and surface stain.
Despite the remarkable effectiveness of the plasma treatment with the above described object, the method sometimes suffers from poor reproducibility of the results for an unknown reason presenting a great drawback to the practical application of the method.